ARO Trust (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Marketable Securities [Line Items]
Debt and Equity Securities, Cost	$ 198.8	$ 197.6
Debt and Equity Securities, Fair Value	282.1	268.9
External trust [Member]
Marketable Securities [Line Items]
Funding obligation	16.0
Money Market Funds [Member]
Marketable Securities [Line Items]
Debt and Equity Securities, Cost	26.7	25.5
Debt and Equity Securities, Fair Value	26.7	25.5
U.S. Equity Funds [Member]
Marketable Securities [Line Items]
Debt and Equity Securities, Cost	52.7	52.7
Debt and Equity Securities, Fair Value	130.7	119.8
International Equity Funds [Member]
Marketable Securities [Line Items]
Debt and Equity Securities, Cost	31.7	31.7
Debt and Equity Securities, Fair Value	40.9	39.2
Municipal Bond Funds [Member]
Marketable Securities [Line Items]
Debt and Equity Securities, Cost	87.7	87.7
Debt and Equity Securities, Fair Value	$ 83.8	$ 84.4

Amount of contractual obligation, including, but not limited to, long-term debt, lease obligation, purchase obligation, and other commitments.

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Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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